1) Field of the Invention
The present invention relates to a technology that can suppress noise produced by various motors or moving mechanical parts in an image formation apparatus.
2) Description of the Related Art
In recent years, there is a growing request for suppressing unpleasant noise produced by the office automation (OA) equipment in the offices. Noiseless office automation equipment have appeared in market. Such equipment improve the working environment in the offices as they do not produce noise.
A technique for suppressing noise is disclosed in, for example, Japanese Patent Application Laid-open Publication No.9-193506. This publication discloses a noise masking apparatus for the laser beam printers or the copying machines. This noise masking apparatus includes a sound generator that generates masking sound to mask noise produced by the motors or the moving mechanical parts. Moreover, a masking sound controller controls the sound generator to generate an appropriate masking sound. The masking sound is of a frequency that includes the frequency range of the main component of the noise. In this technique, however, as the masking noise is added to the noise, sometimes the noise level raises. Moreover, additional space is required to accommodate the sound generator and the masking sound controller.
It is common to use the acoustic power level (ISO7779) to evaluate noise in the office automation equipment. However, the acoustic power level is the value of the acoustic energy produced by the office automation equipment and has a little correlation with human subjective discomfort about noise. For example, even if two sounds have same acoustic power level, a listener may feel only one of them to be noisy. Moreover, some listener may feel that the sound is noisy even if the acoustic power level is low.
Therefore, the better approach will be to improve the sound quality in addition to lowering the acoustic power level of the office automation equipment. To improve the sound quality, it is necessary to quantitatively measure the sound quality before and after the improvement. However, as the sound quality is not physical parameter, it can not be measured quantitatively. In other words, evaluation of sound quality is different depending on persons. Only qualitative expression is possible such as “the sound quality is a little improved” or “the sound quality is substantially improved”. When the sound quality cannot be quantitatively expressed as physical characteristics, the effect of a sound quality improvement measure taken cannot be subjectively evaluated. Therefore, it is necessary to carry out a subjective evaluation experiment and statistically quantify the sound quality based on a result of this experiment.
A psycho-acoustics parameter is available as a physical level to evaluate sound quality. The following representative parameters are available. Refer to “The seventh design optic system lecture” of the Japan Society of Mechanical Engineers, “Targeting at an innovative jump of design and system toward the twenty-first century!”, Nov. 10, 1997, “Sound, oscillation, design, color, and design (1)”, 089B. Units are expressed in brackets.                (1) Loudness (sone): loudness of sound        (2) Sharpness (acum): relative distribution of higher harmonic components        (3) Tonality (tu): tonality, ingredient of pure tone component        (4) Roughness (asper): roughness of sound        (5) Fluctuation strength (vacil): variable strength, beat tone        (6) Impulsiveness (iu): impulsiveness        (7) Relative approach: sensation of fluctuation        
An increase in each of the above psycho-acoustics parameters tends to increase discomfort. Only the loudness is standardized by the ISO532B. The rest of the psycho-acoustics parameters have the same idea in principle. However, as programs and calculation methods are different depending on individual researches carried out by each maker, measurement values are usually slightly different between makers. It is know that the sound quality can be improved if all these psycho-acoustics parameters are lowered.
Substantial work is necessary to take measures for all the psycho-acoustics parameters. Noise generated from office automation equipment such as a copying machine and a printer includes noise of various tones because of the complexity of the mechanism. For example, somber noise of low frequency shrill noise of high frequency, and impulsive noise are generated while changing from a plurality of noise sources such as a motor, a recording paper, a solenoid, etc.
A person judges these noises as a whole, and determines whether they are unpleasant. The person is considered to make a judgment by weighting a particularly unpleasant element. In other words, psycho-acoustics parameters highly related to discomfort and psycho-acoustics parameters not highly related to discomfort are present. These are different according to tones of machine. For example, when a printer rotates at a high speed and generates impulsive noise many times, impulsive noise is felt most unpleasant. A quiet desktop printer of a relatively low speed gives little impulsive noise. Therefore, charging noise generated when the printer is charged with alternating current (hereinafter, “AC”) power is most unpleasant. Unpleasant noise tones are different depending on the output speed of the image formation apparatus. Consequently, tones that require improvement in sound quality are different between low-speed apparatuses and high-speed apparatuses. Therefore, when psycho-acoustics parameters having a large effect of improvement from discomfort are found and improved thereby to efficiently improve the sound quality, trials and errors for the improvement need not be repeated.
Therefore, psycho-acoustics parameters having a large effect of improvement from discomfort are combined together, and these psycho-acoustics parameters are weighted to calculate a sound quality evaluation expression. This sound quality evaluation expression is used to calculate a subjective evaluation value for discomfort. With this arrangement, subjective evaluation of sound quality becomes possible, which improves sound quality. Further, a discomfort subjective evaluation level at which sensation of discomfort is not present is decided. The sound quality is improved to a level not more than this value. When an image formation apparatus that achieves the above is provided, it is possible to solve the noise problems in the office.
The inventor(s) of the present invention obtained sound quality evaluation expressions. These expressions correspond to a copying apparatus of a low copying speed (i.e., printing speed) of 16 to 20 ppm, (where ppm represents a printing speed for A4 horizontal size paper) a copying apparatus of an intermediate copying speed of 27 ppm, and a copying apparatus of high copying speed of 45 to 70 ppm respectively. The inventor(s) have filed an application for patent for these sound quality evaluation expressions.
When the copying apparatus has the printing speed of 16 to 20 ppm, the discomfort is expressed using loudness (size of audible level) and tonality (relative distribution of pure tone component) according to subjective evaluation experiments and multiple regression analysis. Discomfort exponent S given by:   S  =            0.3135      ×              (        loudness        )              +          3.4824      ×              (        tonality        )              -          3.146      ⁢                           ⁢              (                              -            1                    ≤          S          ≤          1                )            fulfills the inequality S<−0.6.
When the copying apparatus has the printing speed of 45 to 75 ppm, the discomfort exponent S is expressed using a squared loudness and sharpness (relative distribution of high-frequency component) according to subjective evaluation experiments and multiple regression analysis.                     S        =                ⁢                              0.01024269            ×                                          (                loudness                )                            2                                +                                                ⁢                              0.30996744            ×                          (              sharpness              )                                -                      2.1386517            .                              
When the copying apparatus has the printing speed of 27 ppm, the discomfort exponent S is expressed using a sound pressure level and sharpness (relative distribution of high-frequency component) according to subjective evaluation experiments and multiple regression analysis.   S  =            0.0931      ×              (                  sound          ⁢                                           ⁢          pressure                )              +          0.5254      ×              (        sharpness        )              -    6.1935  
However, as described above, there are the three kinds of sound quality evaluation expressions, as the portions that are felt unpleasant are different depending on the copying apparatus of the low copying speed (i.e., printing speed) of 16 to 20 ppm, the copying apparatus of the intermediate copying speed of 27 ppm, and the copying apparatus of the high copying speeds of 45 to 70 ppm respectively. Consequently, it is not possible to effectively evaluate the sound qualities of all of these apparatuses.
In other words, as the sound quality evaluation values calculated according to these sound quality evaluation expressions have no unit because of their predictive values for evaluating sound based on a subjective comparison of sound. The sound quality evaluation values work within a range of the subjective evaluation experiments. Therefore, as a matter of fact, discomfort levels are different even when the sound quality evaluation values are the same. For example, even when the value calculated according to the low-speed-layer sound quality evaluation expression and the values calculated according to the intermediate and high-speed-layer sound quality evaluation expressions are both “0”, the discomfort levels are not the same.
Further, the inventor(s) of the present invention integrated the above three expressions corresponding to respective speeds into one sound quality evaluation expression in the early application. In other words, the inventor(s) obtained the following multiple regression equation having no constant term by using psycho-acoustics parameters as explanatory variable according to the Scheffe's method of a paired comparison method, in order to predict a difference (Ai−Aj) of an average discomfort effect of sample sounds Ai and Aj.                                                                                           α                  ⁢                                                                           ⁢                  i                                -                                  α                  ⁢                                                                           ⁢                  j                                            =                            ⁢                                                0.2307484                  ⁢                                                                           ⁢                                      (                                                                  x                        ⁢                                                                                                   ⁢                        loudness                        ⁢                                                                                                   ⁢                        i                                            -                                              x                        ⁢                                                                                                   ⁢                        loudness                        ⁢                                                                                                   ⁢                        j                                                              )                                                  +                                                                                                      ⁢                                                0.3720474                  ⁢                                                                           ⁢                                      (                                                                  x                        ⁢                                                                                                   ⁢                        sharpness                        ⁢                                                                                                   ⁢                        i                                            -                                              x                        ⁢                                                                                                   ⁢                        sharpness                        ⁢                                                                                                   ⁢                        j                                                              )                                                  +                                                                                                      ⁢                                                4.3095786                  ⁢                                                                           ⁢                                      (                                                                  x                        ⁢                                                                                                   ⁢                        tonality                        ⁢                                                                                                   ⁢                        i                                            -                                              x                        ⁢                                                                                                   ⁢                        tonality                        ⁢                                                                                                   ⁢                        j                                                              )                                                  +                                                                                                      ⁢                              1.2007391                ⁢                                                                   ⁢                                  (                                                            x                      ⁢                                                                                           ⁢                      impulsiveness                      ⁢                                                                                           ⁢                      i                                        -                                          x                      ⁢                                                                                           ⁢                      impulsiveness                      ⁢                                                                                           ⁢                      j                                                        )                                                                                        (        1        )            This multiple regression equation (1) is modified into an expression for obtaining a relative evaluation point of the sample sound Ai.
A person who is doing the evaluation (hereinafter, “evaluating person”) gives −1 as an evaluation point when Ai is more unpleasant than Aj, and gives 1 as an evaluation point when Aj is more unpleasant than Ai. Therefore, only values from −1 to 1 can be taken as an average discomfort effect (i.e., measured value) according to the experiments. However, as the multiple regression equation (1) is a linear model, the prediction value of the discomfort effect by calculation is smaller than −1 or is larger than 1 depending on the input psycho-acoustics parameter value. Consequently, there remains irrationality that a range of actual measured values and a range of predicted values are different as indicated by ellipses in FIG. 5.
Further, the evaluation result according to the Scheffe's method of the paired comparison method is the obtaining of a subjective distance between discomforts of sample sound. The relative evaluation point expressed by the multiple regression equation (1) takes a range from −1 to 1. However, as the numerical values take no unit, an improvement of 0.2 from discomfort, for example, does not indicate a specific level of improvement.